Topic 16 · page 18 of 19
Fuel cells
A fuel cell turns the energy released when a fuel reacts with oxygen into a voltage. Fuel and oxygen are supplied continuously, so it does not run flat.
Read first: Predicting feasibility
What a fuel cell does 16.18
Burning a fuel releases energy mainly as heat. A fuel cell runs the same overall reaction in two half-cells, so the electrons travel round an external circuit and the energy is released as electricity.
Because the reactants are supplied from outside, a fuel cell does not run flat. A battery has its chemicals stored inside and stops when they are used up.
| Feature | Battery | Fuel cell |
|---|---|---|
| chemicals | stored inside the cell | supplied continuously from outside |
| when it stops | when the reactants are used up | when the supply stops |
| recharging | some types can be recharged | not needed: refuel instead |
The energy is released as electricity, not mostly as heat, so a fuel cell is more efficient than burning the same fuel to produce heat and then converting it.
Fuels 16.18
The spec expects you to know that hydrogen and hydrogen-rich fuels such as methanol (CH₃OH) are used.
Hydrogen gives only water as a product. But hydrogen is a gas, so it is difficult to store and to transport: it must be compressed or cooled. Methanol is a liquid, which makes it easier to store and to transport. The overall reaction in a methanol fuel cell is the same as burning it:
How clean is a fuel cell? 16.18
At the point of use, a hydrogen fuel cell gives out only water. How clean the whole process is depends on how the hydrogen was made. Most hydrogen today is made from methane, which releases carbon dioxide. Hydrogen made by electrolysis is only as clean as the electricity used. A fuel cell is therefore only as clean as its fuel.
Check yourself
- What does a fuel cell do?
- How does a fuel cell differ from a battery?
- Give one reason why methanol is used in some fuel cells instead of hydrogen.
- Write the overall equation for a methanol fuel cell, with state symbols.
- A newspaper says hydrogen cars are "zero emission". Comment, using what you know about hydrogen fuel cells.
Answers cover these until you have tried the questions
- It uses the energy released when a fuel reacts with oxygen to generate a voltage, with the fuel and oxygen supplied continuously.
- The reactants of a fuel cell are supplied continuously from outside, so it does not run flat or need recharging. A battery has its chemicals stored inside and stops when they are used up.
- Methanol is a liquid, so it is easier and safer to store and transport than hydrogen gas.
- At the exhaust only water is produced, so at the point of use it is true. But most hydrogen is made from methane (releasing CO₂), and hydrogen from electrolysis is only as clean as the electricity used. So the whole process is not necessarily zero emission.